Austenitic steel alloy



United States Patent 3,113,861 AUSTENITIC STEEL ALLOY Telier E. Norman, Denver, Colo., assignor to American Metal Climax, Inc, New York, N.Y., a corporation of New York No Drawing. Filed hiay 16, 1961, Ser. No. 110,337 7 Claims. (Cl. 75-123) The present invention resides in novel alloy steels and in particular to high carbon austenitic steels characterized by high abrasion resistance, combined with good ductility, good impact resistance and a rapid rate of workhardening when plastically deformed in service.

Cast or wrought steels for applications requiring a high degree of abrasion resistance are commonly made from compositions containing about 0.50 to 1.50% carbon, together with suitable alloy additions and heat treatments to produce certain desired combinations of hardness, strength, ductility and wear resistance. The microstructures existing in these steels may generally be classed as pearlitic, martensitic or austenitic or in some cases combinations of these microstructures may exist in certain abrasion resistant parts.

For a high carbon steel of a specific composition, greatest resistance to abrasion is normally obtained when its structure is made martensitic, which produces the maximum hardness obtainable from the composition. However, I have found that high carbon austenitic steels will have abrasion resistance which approaches that of martensitic steels of equivalent carbon content, provided the austenitic structure is sufiiciently unstable so that it work hardens rapidly on its wearing surface when plastically deformed. Substantial amounts of suitable alloying elements are necessary in any high carbon steel in order for it to retain a fully austenitic structure at normal atmospheric temperatures. For example, in a steel containing 1.20% carbon, it requires a minimum of about 4.0% manganese, or about chromium, or about 8% nickel or various combinations of these alloying elements in order to retain a fully austenitic structure at normal atmospheric temperatures. While each of these three alloying elements are capable of retaining an austeni-tic structure in a high carbon steel, they produce different effects on the relative toughness, work hardening characteristics and abrasion resistance of the austenitic structure. Molybdenum additions in amounts up to about 3.0% may also be used in combinations with other alloying elements to assist in retaining austenite in high carbon steels. The effect of molybdenum in these amounts is principally to assist in retaining the carbon in the austenite in solution, and this carbon in solution in turn has a potent effect in retaining an austenitic structure in a suitably heat treated steel.

In my present invention I have taken advantage of the various effects of austenite-retaining elements to produce a high carbon austenitic steel whose abrasion resistance approaches that of high carbon martensitic steels.

The austenitic steel of my invention has much greater ductility and impact resistance and is much easier to heat treat free from quench cracks or high residual stresses than high carbon martensitic steels. A further advantage of the alloy of my invention is that its structure and properties are obtainable by the use of relatively moderate amounts of alloying elements, so that it can be produced for an economical and competitive cost.

The chemical composition range of the alloy of my invention, in percent by weight, is as follows:

Carbon 1.00l.50

Manganese 4.509.00 Silicon 0.20-0.90

Molybdenum 0.25-3.00

Carbon -1 .40 Manganese 5 .007.00 Silicon 0.30-0.70 Molybdenum 0504.50 Phosphorus 0.05% max. Chromium 0.50% maX.

In the foregoing preferred composition, the manganese may be held near the low side of the range when maximum abrasion resistance is desired, or the manganese may be held near the high side of the range when best impact resistance and improvement in other mechanical properties is desired.

.The molybdenum content in the foregoing preferred composition may be held near the low side of the range in light-section, rapidly quenched parts and should be near the high side of the range in parts having cross section thickness greater than about 4 inches. When the alloy is used in a foundry to produce castings in a variety of sizes and thicknesses, a molybdenum content of about 0.8 to 1.20% represents 'a desirable and practical range.

The alloy of my invention is made fully austenitic by heating it to a temperature of 1900 F. to 2-000 F. for a sufficient length of time to dissolve all or most of the carbon into solid solution in the austenitic matrix, after which it is quenched rapidly in Water or other suitable liquid, or in the case of light section parts which can be lair-cooled rapidly, it may be air quenched. An alternative treatment for certain castings is to remove the castings from their molds after they have solidified, but are still above about 1800 F then quench them directly into Water or other suitable quenching media. This direct quench procedure is particularly adaptable to castings which have been made in permanent metal or graphite molds or which have been cast as bars or rods in a continuous casting process. A modification and improvement of the direct quench procedure is to remove the castings from their molds soon after casting and solidfication, then place them in a furnace held at 1900 F hold them in this furnace until the castings are uniformly at this temperature throughout their cross section, after which they are removed and quenched in the usual manner.

In order to study and evaluate various types and compositions of steel for abrasion resistant applications, I have developed a practical test which has been used in the ore grinding operations of Climax Molybdenum Company at Climax, Color-ado, to evaluate and compare the relative abrasion resistance and impact resistance of various types of steel. For this test the steels to be studied are cast or forged into large grinding balls 4 to 5 inches in diameter. After being given the desired heat treatment, they are marked with suitable identification marks and are then charged simultaneously into a large commercially operating ball mill where the test balls are exposed to the normal abrasive and tumbling action in this mill for periods ranging from one to two weeks duration. The test balls are recovered from the mill after each test period and their weight losses and relative wear rates are determined. Also their susceptibility to breakage or spalling in service during the test is observed. To provide a standard of comparison, a group of high carbon martensitic steel balls having a specific composition and treatment is always included with the groups of test balls. Two

or more balls having the same composition and treatment are used for each group. This test was developed principally to evaluate materials for possible subsequent use in grinding mill liners but it also serves as a guide or indiimprovement in wear resistance in these austenitic manganese steel-s. However, the principal function of molybdenum in the alloy of my invention is to suppress or prevent precipitation of embrittling types of carbides around cator of abrasion resistance and toughness for other ap- 5 grain boundaries or along crystallographic planes. When plications. The test is described in greater detail in my molybdenum is not present in the composition, these emtechnical paper entitled Factors Influencing the Resistbrittling types of carbides tend to occur, especially in the ance of Steel Castings to High Stress Abrasion, which heavier sections of high carbon, lean alloy austenitic man- Was published in Modern Castings for May 1958, pages ganese steels. A major virtue of the alloy is that it is duc 89 to 98, and in the Transactions of the American Foundtile and Work hardens to an important degree under imrymens Society, volume 66 (1958), pages 187 to 196. pact.

In the above mentioned AFS paper I also pointed out Some of the advantages of the alloy of my invention that my wear tests indicated the lean alloy types of may be summarized as follows: austemt? had much better abraslon Teslstance than i (1) It has substantially better wear resistance than the conventional Hadlield type 12% manganese austenitic 15 Hadfielld type 12% manganese austenitic Steels. However It was also Pomted t mllny of (2) It has a more rapid rate of work hardening and the lean alloy types of austenite had 1nsufiic1ent impact greater resistance to flow when exposed to repeated resistance or toughness for commercial use. The alloy impact than the Hadfigld type 12% mamanese of the present invention provides the desired combination austenitic steels offillghness abraslm} reslstance' (3) Its abrasion resistance approaches and in some cases e following table lists the observations and results p equals the abrasion resistance of high carbon mar- Of .wear'lmp'act test? mechamcal. property on a tensitic steels, while at the same time it has substanseries of 24 austenitic steel compositions. In this table tiany greater ductility and impact resistance than the the abrasion factors represent rates of wear relative to a high carbon martensitic Steels hi 8 g rfi a n 21 1233 2 1 3 ill f nifil ih zl l if; 25 Cost of alloying elements in the steel is moderate, its 1 o g P heat treatment is relatively simple and no special or ybdenum. Low factors indicate good wear resistance I unusual types of equipment are required for its proand high factors indicate inferior wear resistance. duction and heat treatment gg iig z 3 g; f f gi figfi l g gf gg (5) It has a combination of tensile strength, impact y g 0 strength, ductility and abrasion resistance which make ductlhty or Impact reslstance the w-ldely used Hadfield it permissible and attractive for use in a wide variety type 12% .mallganese Steel (Item 11 tab1e).there are of applications involving abrasive service many applications Where the full ductility and nnpact resistance of the Hadfield type steel is unnecessary. It is What is claimed is: in such applications that the alloy of my invention, with 1. An alloy steel consisting of 1.0 to 1.5% carbon, 4.5 its substantially superior abrasion resistance, is commerto 9.0% manganese, 0.2 to 0.9% silicon, 0.25 to 3.0% cially attractive. molybdenum, the remainder essentially iron.

It Composition, Percent {22% Tensile Etlonga- Ab 1 ie ion, ras on $0 Stu, p.s.i. Percent Factor 0 Mn Si Or Mo Ni Cu p.s.i.

6. 0 0. 5 56, 800 83,700 15.0 114 5.6 0. 5 54, 650 72, 850 9. 5 115 5. 5 0. 5 52,150 71,750 10. 5 116 5. 6 0. 5 50, 550 73, 300 7. s 117 5. 6 1.1 54, 350 76, 400 8.8 119 5. 6 1. 5 56, 700 79, 250 8.8 121 5. 5 0. 5 51, 500 76,500 9. 3 122 1. 5 0. 5 58,700 84, 000 9. 5 127 12. 0 0. 5 68, 400 106, 900 18. 0 123 5. 7 0. 5 53,500 80, 750 12.8 129 3-? as 388 2 2 3:0 05 0.5 (hacked onquenehing broke 5.9 0.5 3.0 0.5 66,100[ 87,500 10.0 broke 3.0 0.5 3.1 0.5 67,100 82,600 4.5 broke I. 5 3.0 0. 5 cracked on quenching broke 1.5 0.5 3.0 0.5 59,900 67,800 4.0 broke 1.5 0.5 3.0 0.5 59,900 72,300 3.0 broke 1.5 0.5 3.0 0.5 63,000 72,700 2.5 broke 2.4 0.5 3.0 0.5 58,300 72, 700 5.0 broke 2.4 0.5 0.5 56,800 70,400 5.0 broke 1.5 0.5 3.0 0.5 02,000 75, 300 3.5 broke 1. 5 0. 5 3.0 1. 0 cracked on quenching broke I. 5 0. 5 3.0 2.0 cracked on quenching broke 1 The termbroke refers to breakage during the abrasion tests from which the abrasion factors were calculated.

The results presented in the table indicate that many austenitic steel compositions may not have sufiicient toughness for use in commercially produced steel castings. In particular the addition of 3% chromium to many of the compositions in the table produced harmful effects. This chromium addition to the various steels produced carbide precipitates along crystallographic planes and around grain boundaries, which in turn were responsible for the cmbrittlement of these compositions. A high silicon content acts similarly and was responsible for the breakage of item 12. Furthermore it is known from other investigations that phosphorus contents in excess of about 0.06% tend to produce serious loss of ductility and impacts resistance in all types of high carbon austenitic steels.

Molybdenum additions of 0.5 to 1.0% provide a small 2. An alloy steel consisting of 1.0 to 1.5% carbon, 4.5 to 9.0% manganese, 0.2 to 0.9% silicon, 0.25 to 3.0% molybdenum, not over 0.08% phosphorus and not over 1.0% chromium and the remainder iron.

3. An alloy steel consisting of 1.2 to 1.4% carbon, 5.0 to 7.0% manganese, 0.3 to 0.7% silicon, 0.5 to 1.5% molybdenum, not over 0.05% phosphorus, not over 0.5% chromium and the remainder essentially iron.

4. An austenitic alloy steel consisting of 1.0 to 1.5% carbon, 4.5 to 9.0% manganese, 0.2 to 0.9% silicon, 0.25 to 3.0% molybdenum, the remainder essentially iron.

5. An alloy steel consisting of 1.0 to 1.5% carbon, 4.5 to 9.0% manganese a minimum of 0.2% and a maximum of 0.9% silicon, 0.25 to 3.0% molybdenum, a maximum D of 0.08% phosphorus, a maximum of 1.0% chromium, and the remainder essentially iron.

6. A cast alloy consisting of 1.2% to 1.4% carbon, 5.0 to 7.0% manganese, 0.3 to 0.7% silicon, 0.8 to 1.20% molybdenum, and the remainder iron.

7. A cast alloy consisting of 1.2 to 1.4% carbon, 5.00 to 7.00% manganese with the manganese content near the low end of the range when maximum abrasion resistance is desired and near the high end of the range When maximum impact resistance is desired, 0.3 to 0.7% silicon, 0.5 to 1.50% molybdenum with the molylbdenum content near the low end of the range in relatively thin section parts having a cross-sectional thickness lower than approximately four inches and near the high end of the range in relatively heavy section par-ts having a crosssectional thickness higher than approximately four inches, and the remainder iron.

References Cited in the file of this patent UNITED STATES PATENTS 

1. AN ALLOY STEEL CONSISTING OF 1.0 TO 1.5% CARBON, 4.5 TO 9.0% MANGANESE, 0.2 TO 0.9% SILICON, 0.25 TO 3.0% MOLYBDENUM, THE REMAINDER ESSENTIALLY IRON. 